This invention relates to detection of a particular signal sequence in a received signal which is produced by angular modulation of a modulating signal comprising symbols representative of binary one and zero at a predetermined symbol interval. The particular signal sequence is given by subjecting to the angular modulation a particular symbol pattern which the modulating signal comprises and which consists of a predetermined number of symbols in a predetermined manner. Typically, the particular symbol pattern is a preamble which precedes an information signal of the modulating signal and is used in time division multiple access (TDMA) communication.
It will be assumed for the time being that detection of the particular signal sequence is used in a time division multiple access radio communication network. In other words, a method or a device for detecting the particular signal sequence is used in a receiver included in the network. The particular symbol pattern is preliminarily known by an attendant to the receiver.
Inasmuch as the received signal is subjected to the angular modulation, the receiver comprises a coherent detector for subjecting the received signal to coherent demodulation or detection to produce inphase (I) and quadrature phase (Q) analog signals. It is possible to understand that the inphase and the quadrature phase analog signals are cooperative to define a signal vector on a phase plane defined by orthogonal I or real and Q or imaginary axes. The signal vector has a phase which is variable in accordance with the modulating signal. The coherent detection therefore produces in short the signal vector or a vector signal representative of the signal vector.
According to prior art, the particular signal sequence is detected by using the inphase and the quadrature phase analog signal in regenerating a clock signal of a clock period which should be equal to the symbol interval. The clock signal is used in determining a time instant of discrimination of the particular symbol pattern. Before detection of the particular signal sequence, such time instants are used in regenerating a regenerated datum from the inphase and the quadrature phase analog signals. If the regenerated datum includes a portion coincident with the particular symbol pattern, it is known that the received signal includes the particular signal sequence.
When the particular signal sequence should be detected in the radio communication network, the received signal is inevitably subjected to multipath or multi-path transmission which gives rise to intersymbol interference in the regenerated datum. This brings forth an adverse influence on the regenerated datum and consequently adversely affects adequate detection of the particular signal sequence.
It is known in the art to use transmission path equalization in order to get rid of the adverse influence and thereby to enable correct regeneration of the datum and correct detection of the particular signal sequence. A considerably long time is, however, necessary to detect the particular symbol pattern and the particular signal sequence when the transmission path equalization is resorted to. This is objectionable particularly in high-speed digital communication.
Even when the particular signal sequence should be detected in a wired or an optical communication network, the clock signal may not always be regenerated with a correct clock period. In this event, it becomes impossible to correctly obtain the regenerated datum. As a consequence, the particular signal sequence can not adequately be detected.